This invention relates to a fluidized bed steam generating system and, more particularly, to such a system which includes a separate fluid flow circuitry between the furnace section and the separating section.
Fluidized bed combustion systems in connection with separators are well known. In these arrangements, air is passed through a bed of particulate fuel, possibly coal, wood or dehydrated sewage sludge, to fluidized the bed, and thereby, effectuate high combustion efficiency at a relatively low temperature. This process, however, results in flue gases which retain a large amount of fine particulates. The gas stream is therefore passed into a separator which separates the particulates from the gas and recycles them back into the bed.
In conventional steam generating systems, the passage between the furnace and the separator is usually defined by a relatively expensive, high temperature, refractory-lined duct due to the extreme temperature of the flue gases.
This duct is either left relatively thin due to the expense and weight of the refractory material which results in excessive heat losses to the environment, thereby reducing the system's efficiency, or it is made relatively thick which adds to the bulk, weight and cost of the separator. Even when the duct is thick, all the heat losses cannot be prevented since perfect insulation would raise the duct's temperature to an unacceptable degree.
A further problem associated with the use of a refractory-lined duct is the lengthy time required to warm the walls before putting the system on line to eliminate premature cracking of the refractory material. This lengthy delay is inconvenient and adds to the cost of the process.
For relatively small steam generating systems, these problems can be prevented by forming the duct directly out of the walls of the furnace and separator. This is accomplished by bending a plurality of cooling tubes of each device out of their planes to form both an outlet and inlet which can be welded together. This process is not feasible in larger systems due to the engineering requirement that the duct leading into the separator be several feet in length in order to maintain an acceptable separator collection efficiency. Further, this process is complex and expensive due to the elaborate bending patterns required.